Dichroism

Dichroism is the polarization-dependent absorption of light by a material. It can make a material appear different colors under different polarization directions.

Linear dichroism: A difference in light absorption between two perpendicular linear polarization directions. It is the form of dichroism associated with absorption that varies across linear polarization directions.
Polarization: The orientation and pattern of a wave’s transverse oscillations. Dichroism compares absorption across distinct polarization states.
Circular dichroism spectroscopy: A spectroscopic technique that measures differential absorption of left- and right-circularly polarized light. It uses circular dichroism to investigate molecular structure and chirality.
Optical activity: The rotation of a material’s effect on the orientation of linearly polarized light, often through circular birefringence. It is another polarization-dependent optical response, distinct from differential absorption.
Circular dichroism: A difference in a material’s absorption of left- and right-circularly polarized light. It extends polarization-dependent absorption from linear to circular polarization.
Chromophore: A molecular group that absorbs particular wavelengths of light, often giving a substance its color. Oriented chromophores can absorb one polarization more strongly than another.
Linear dichroism spectroscopy: A technique that measures absorption differences for linear polarizations to study molecular alignment. It turns linear dichroism into a probe of oriented molecules and materials.
Cotton effect: A sharp change in optical rotation and circular dichroism near an absorption band of a chiral substance. It is a characteristic spectral feature arising where circular dichroism is strong.
Birefringence: The dependence of a material’s refractive index on light polarization or propagation direction. It also distinguishes polarization directions, but through refraction rather than absorption.
Molecular orientation: The preferred alignment of molecules or molecular groups within a material. A common source of unequal absorption along different polarization directions.

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